Just to update this. Curiosity got the better of me and I fitted the eibach springs a few months ago with the intention of removing them again if I or my family found them too harsh but in actuality no one has noticed that I fitted them since.
They lowered the car bang on 30mm all round and leave it sitting perfectly level unlike the factory springs which left the front of the car higher (bigger tyre gap) than the rear. It looks absolutely perfect now.
I am convinced that Mazda cheaped out and fitted the same shocks/springs to both petrol and diesel cars from 2010 onwards. A mate of mine has a 2011 2.2 (180ps) Sport and parked beside mine, his front was 11mm lower than mine on factory springs and his had similar low mileage to mine and so his shocks/springs were in good order.
I'm also convinced that pre face lift cars had different springs/shocks as they all appear to sit slightly lower in the front than mine did. I hated the way mine looked and it was really spoiling the car for me.
Since fitting the eibach springs (which are specific to petrol models (the front springs are rated for 140kg less than the diesel version, rear springs are the same for both models)) it seems as if the eibach springs are better matched to the shocks than the factory springs. The car is so much smoother now, the front doesn't dip and rise on braking/acceleration and doesn't get unsettled on humps/bumps on the road as it done previously.
Comfort wise there is no difference, on poorer surfaces there is obviously less suspension travel but it hasn't compromised comfort and there are no issues with clearance.
It handles very well, even the simple things like on roundabouts the car is more stable with less roll.
I fitted mine myself along with a mate, it took less than 2hrs. Earlier that day, I sprayed all links/bolts with WD40 and so they all came off relatively easy. The rears took less than 20 minutes to do both. I just dropped the roll bar (removed the bushings not the links) and removed the large front bolt and swung the whole arm down, the springs pops out easily. The fronts were also easy to do just took a little longer.
Overall very impressed with them and would recommend them to anyone looking to get rid of the ridiculously large wheel gap on the Gen2 mazda 6 without compromising comfort or clearance.